598
2014, 59, nr 7—8
Modification of hydrolytic stability of high molecular
weight poly(e-caprolactone) by addition of medium
molecular weight poly(e-caprolactone) aggregated
in the presence of CaO (Rapid Communication)
Melania Bednarek1),*), Micha³ Cichorek1)
DOI: dx.doi.org/10.14314/polimery.2014.598
Abstract: Medium molecular weight poly(e-caprolactone) containing three carboxyl groups at one chain
end (PCL5000) and the same polymer but aggregated in the presence of CaO (PCL5000/CaO) were used
as an additive for modification of hydrolytic stability of high molecular weight poly(e-caprolactone)
(PCL80000). Thermal and mechanical properties of PCL80000 and its composites together with their
molecular weight changes upon hydrolysis were investigated. The composite obtained using
PCL5000/CaO showed higher tendency to hydrolytic degradation than unmodified PCL80000 with the
preservation of mechanical properties.
Keywords: biodegradable polyester, poly(e-caprolactone) modification, composites, hydrolytic degradation.
Modyfikacja stabilnoœci hydrolitycznej poli(e-kaprolaktonu) o du¿ym ciê¿arze cz¹steczkowym za pomoc¹ zagregowanego w obecnoœci CaO poli(e-kaprolaktonu) o œrednim ciê¿arze cz¹steczkowym
Streszczenie: Poli(e-kaprolakton) o œrednim ciê¿arze cz¹steczkowym (PCL5000) zawieraj¹cy trzy grupy
karboksylowe na jednym koñcu ³añcucha oraz ten sam polimer zagregowany w obecnoœci CaO
(PCL5000/CaO) zastosowano jako domieszki w celu modyfikacji stabilnoœci hydrolitycznej wielkocz¹steczkowego poli(e-kaprolaktonu) (PCL80000). Zbadano w³aœciwoœci termiczne i mechaniczne PCL80000
oraz jego kompozytów, a tak¿e zmiany ciê¿aru cz¹steczkowego w wyniku ich hydrolizy. Kompozyt
otrzymany z dodatkiem PCL5000/CaO wykaza³ wiêksz¹ podatnoœæ na hydrolizê ni¿ niemodyfikowany
PCL80000, bez pogorszenia w³aœciwoœci mechanicznych.
S³owa kluczowe: poliester biodegradowalny, modyfikacja poli(e-kaprolaktonu), kompozyty, degradacja
hydrolityczna.
An important property of degradable polymers is
their ability to undergo hydrolytic or enzymatic degradation to low molecular weight compounds [1, 2]. For
diverse potential applications different degradation
rates may be advantageous. Thus, there are numerous
studies in the literature concerning factors affecting
the degradation rate. The most important class of
degradable polymers is aliphatic polyesters [1].
Among them, polylactide (PLA) is most extensively
studied as a prospective degradable polymer in packaging, agriculture and biomedical applications. One of
the approaches to modifying its properties, including
the degradation rate, is based on blending it with inor-
Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, ul. Sienkiewicza 112, 90-363 £ódŸ, Poland.
*) Author for correspondence; e-mail: [email protected]
1)
ganic compounds (mainly calcium salts) [3, 4]. Studies
of such systems revealed their complexity [3, 4], which
may be partly due to the physical form of the added
inorganic salt. Much less is known of the effect of inorganic additives on the degradation rate of another
degradable polyester, namely poly(e-caprolactone)
(PCL).
We have previously reported [5] that medium molecular weight PCL containing different numbers of carboxyl groups at one chain end interacts with calcium cations (introduced as calcium oxide) forming aggregates
in solution, as evidenced by dramatic increase in solution
viscosity (from 150 cP up to about 1·106 cP). Finally, viscous solutions turned into transparent gels. After solvent
evaporation, solid PCL material was obtained, in which
long cylindrical structures (formed probably on aggregate templates) were present as it was found on the basis
of SEM analysis [5]. By the incorporation of CaO in aggre-
POLIMERY 2014, 59, nr 7—8
gate structures via ionic interactions with -COOH
groups, calcium ions were uniformly dispersed in PCL
matrix.
PCL is a degradable polymer but the rate of its hydrolytic degradation is rather low, which may be a limitation
if biomedical applications are considered. Hydrolysis is
catalyzed by acids, thus blending the high molecular
weight PCL with small amount of PCL oligomers containing relatively higher fraction of terminal -COOH
groups should enhance the rate of hydrolysis. The presence of -COOH groups, however, may have a detrimental effect to mechanical properties of material due to possible degradation during processing. We speculated that
blending PCL-(COOH)3/Ca2+ aggregates with high molecular weight PCL should enhance the rate of hydrolytic
degradation of material when exposed to water without a
deterioration of mechanical properties during preparation. To check this hypothesis, preliminary investigation
of properties of blends of high molecular weight PCL
with medium molecular weight PCL containing Ca2+ ions
dispersed and bounded by ionic interactions was conducted and its results are reported in this communication.
EXPERIMENTAL
Materials
e-Caprolactone (CL, Aldrich, 99 %) was dried over
molecular sieves and distilled on a vacuum line.
Poly(e-caprolactone) (PCL80000, Mn = 80 000, Aldrich)
and citric acid (Aldrich) were dried in vacuum before use.
Triflic acid (98 %, Aldrich) and calcium oxide (p.p.a.,
POCh, Poland) were used as received. 1,2-Dichlorobenzene (Aldrich, 99 %) was distilled.
Preparation of aggregated PCL5000
Poly(e-caprolactone) containing three carboxyl
groups at one chain end [PCL-(COOH)3] with Mn » 5000,
denoted as PCL5000, was synthesized according to the
procedure described earlier [5], by applying cationic
ring-opening polymerization of CL, using citric acid as
an initiator and triflic acid as a catalyst.
An aggregated PCL5000/CaO was prepared by mixing 10 wt % solution of PCL5000 in 1,2-dichlorobenzene
with CaO used with 2-fold excess with respect to -COOH
groups [5].
Preparation of PCL80000/PCL5000 (PCL5000/CaO)
composites
Composites PCL80000/PCL5000 and PCL80000/
PCL5000/CaO were prepared using ZAMAK EHP-5CS
conical twin-screw extruder. PCL80000 was mixed with
6 wt % of PCL5000 or with 6 wt % of PCL5000/CaO for
10 min at 80 °C and 50 rpm.
599
Hydrolysis of PCL80000 and its composites
0.5 g of polymer film was placed in 5 cm3 of distilled
water in vials closed with a lid. The vials were placed in a
thermostated oven at 60 °C for 20 days. The vials content
was then cooled to the room temperature, water was
removed by decantation and the polymer was dried in
vacuum before analysis.
Methods of testing
1
H NMR spectra were recorded in CDCl3 using a Bruker DRX500 spectrometer operating at 500 MHz.
Size exclusion chromatography (SEC) was performed
using an Agilent Pump 1100 Series with an Agilent
G1379A degasser and a set of two PL-Gel 5 µm mixed-C
columns. Wyatt Optilab Rex interferometric refractometer and multi-angle laser light scattering (MALLS)
Dawn Eos laser photometer (Wyatt Technology Corp.,
Santa Barbara, CA) were used as detectors. Dichloromethane was used as an eluent at the flow rate of
0.8 cm3/min at the room temperature. The system was
calibrated according to polystyrene standards. Because
of the discrepancy between molar mass of PCL determined by SEC (MSEC) using polystyrene calibration and
real Mn value [6], molar masses presented as results (Table 1 and 3) were calculated using a correcting coefficient
equal to 0.7 (Mn or Mw = 0.7 · MSEC)
Differential scanning calorimetry (DSC) analysis was
performed under nitrogen at the heating and cooling rate
of 10 °C/min on DSC 2920 Modulated TA Instrument.
Both temperature and heat flows were calibrated with indium. Enthalpy and temperature of melting (DHm and Tm,
respectively) were calculated from DSC traces for the
second run.
Mechanical properties were analyzed using Linkam
TST 353 micro-tensile tester with environmental chamber. For measurements, 0.5 mm thick films were made by
compression molding at 80 °C for 3 min using a hydraulic
press and then rapidly cooled to 15 °C between metal
blocks. Oar shaped specimens complied with the ASTM
D638 (type V) standard with 2× reduced dimensions
(3.81 and 1.50 mm gauge width and length, respectively).
All specimens were uniaxially drawn with 50 %/min
(3.20 mm/s) rate until the break point or slip out from the
machine jaws.
RESULTS AND DISCUSSION
In order to check how the presence of aggregated medium molecular weight PCL containing CaO influences
the properties of PCL with Mn sufficient for application as
a material, the composite of PCL80000 and 6 wt % of
P CL 5 0 0 0 a g g r e g a te d i n the pr e s e n c e of Ca O
(PCL5000/CaO) was prepared by melt mixing of both
components in a counter-rotating twin screw extruder.
The influence of CaO-containing medium molecular
POLIMERY 2014, 59, nr 7—8
600
weight component on PCL physical properties was investigated by comparative studies including SEC and DSC
analysis, tensile tests and susceptibility to hydrolysis performed for PCL80000 alone, PCL80000 containing
unaggregated PCL5000 (PCL80000/PCL5000) and
PCL80000 composite containing aggregated PCL5000
(PCL80000/PCL5000/CaO). Table 1 presents the results of
SEC and DSC analyses.
Fig. 1, and determined values of yield stress (sy), stress
at break (sB) and elongation at break (eB) are listed in
Table 2.
T a b l e 2. Mechanical properties of the studied samples (presented values are calculated as an average of three measurements)
Material
PCL80000a)
PCL80000/PCL5000
T a b l e 1. Characterization of analyzed PCL composites
Mn b)
Material
Mw b)
Mw/Mn
DHm
(DSC)
J/g
Tm
(DSC)
°C
PCL80000a)
81 270 134 500
1.65
62.3
58.2
PCL80000/PCL5000
23 450
57 430
2.44
80.6
57.5
PCL80000/PCL5000/
/CaO
73 080 134 400
1.85
67.3
57.3
a)
PCL80000 was also processed in an extruder in order to provide
comparable processing conditions.
b) Molar mass is given only for the main peak (elution volume
11—17 cm3) which corresponds to macromolecules of PCL80000
overlapping slightly with higher fraction of PCL5000 (PCL5000
alone gives a broad signal at elution volume 16—24.5 cm3).
The blend of PCL80000 with PCL5000 containing free
carboxyl groups undergoes a significant degradation in
the applied mixing conditions (80 °C, 50 rpm, 10 min) as
evidenced by the decrease in Mw and Mn. This may be
attributed to relatively high content of carboxyl groups
catalyzing the degradation. In contrast, blending of
PCL80000 with PCL5000 containing Ca2+ ions does not
cause any detectable degradation.
All three PCL samples were subjected to tensile test on
Instron instrument. Strain-stress profiles are presented in
PCL80000/PCL5000/CaO
sy, MPa
sB, MPa
eB, %
15.0
24.8
1436
—
18.1
20
15.4
27.8
1704
a)
PCL80000 was also processed in an extruder in order to provide
comparable processing conditions.
Different behaviors of starting PCL80000 and
PCL80000 with additives can be observed. PCL 80000 is a
material which during uniaxial drawing attains high
elongation (the limit in the tests was in the range of
1100—1600 % when a specimen slipped out from the machine jaws). The addition of PCL5000 containing free carboxyl end groups causes degradation during processing
which leads to molecular weight decrease and thus decrease of mechanical strength, so that the sample breaks
at relatively low strain and does not achieve the yield
point. Effects of degradation of semi-crystalline polymers
on the degree of crystallinity and brittleness of partly degraded polymers are discussed in detail in a review paper [7]. The addition of aggregated PCL5000/CaO did not
significantly change the PCL80000 tensile properties —
similar values of sy and slightly higher sB were observed
for both materials. The presence of aggregates caused a
slight increase of elongation at break in comparison with
the value for PCL80000. Thus, the involvement of -COOH
30
25
20
20
15
15
10
10
5
5
0
0
0
0
200
400
600
50
100
150
200
800 1000 1200 1400 1600 1800
Elongation, %
Fig. 1. Exemplary stress-strain profiles for the analyzed PCL materials; solid line — starting PCL80000, dashed-dotted line —
PCL80000/PCL5000, dashed line — PCL80000/PCL5000/CaO
5
10
15
20
Elution volume, cm3
25
Fig. 2. SEC curves of PCL polymers after hydrolysis: solid line —
starting PCL80000, dashed-dotted line — PCL80000/PCL5000,
dotted line — PCL80000/PCL5000/CaO (SEC trace of PCL80000
before hydrolysis is shown for comparison, solid thin line)
POLIMERY 2014, 59, nr 7—8
601
groups of low molecular weight PCL in ionic interactions
with Ca2+ cations prevented the composite from deterioration of its mechanical properties which could be caused
by the presence of free carboxyl groups.
At the same time small improvement of elongation at
break was observed which was due to the presence of
medium molecular weight PCL5000/CaO with carboxyl
groups (neutralized with Ca2+ cations) which acted as a
plasticizer.
All prepared PCL samples were subjected to hydrolysis (60 °C for 20 days). Fig. 2 presents SEC curves of the
three analyzed samples and of the starting PCL80000. Determined by SEC analysis values of Mn, Mw and Mw/Mn
are collected in Table 3.
T a b l e 3. Molecular weights of polymers after hydrolysis (20 h
at temp. 60 °C)
Material
PCL80000
PCL80000/PCL5000
PCL80000/PCL5000/CaO
Mn
Mw
Mw/Mn
60 980
104 150
1.71
3640
11 660
3.20
22 440
55 930
2.04
As it may be expected, the presence of PCL5000 containing free carboxyl groups led to a significant increase
of the hydrolysis rate. At the same time, however, the
addition of PCL5000 caused a molecular weight decrease
and thus the decrease of mechanical strength. On the
other hand, the addition of PCL5000/CaO led to a noticeable increase of the hydrolysis rate without affecting the
mechanical properties. Apparently, in bulk, neutralized
carboxyl groups do not catalyze the degradation during
processing while in the presence of water the catalytic
effect is restored. Thus PCL5000/CaO may be treated as a
reservoir of „dormant” carboxyl groups which may, how-
ever, display a catalytic effect in the presence of water. As
a result, the composite material is stable during processing but undergoes enhanced hydrolysis.
CONCLUSIONS
The formation of aggregates upon addition of CaO to
medium molecular weight PCL containing carboxyl end
groups provides a possibility of blending this material
with high molecular weight PCL. The resulting composite shows an enhanced rate of hydrolysis while its
mechanical properties do not deteriorate.
ACKNOWLEDGMENT
Financial support by the National Science Centre (grant
N N204 131940) is gratefully acknowledged.
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Received 30 XII 2013.
Revised version 7 IV 2014.